1. Field of the Invention
This invention relates to an image reading apparatus, and more paticularly an apparatus for reading at least two images different in optical path length from each other which are formed on a plurality of sensors located substantially in the same plane by dividing a light beam from an objective lens in a beam splitter.
2. Description of the Prior Art
Image reading apparatuses of such a type are applicable to a focus state detecting means for cameras. Such a focus state detecting means serves to detect images which have been focused. In order to obtain proper information in the sensors, therefore, it is necessary to divide the imaging light beam while maintaining the imaging performance thereof.
The prior art beam splitters which are required to maintain their imaging performances after dividing are mostly of such an amplitude dividing type that a semipermeable membrane of dielectric material is deposited on a glass substrate. When a single layer of such a semipermeable membrane is actually used, however, the light beam is divided under substantially adverse influence with respect to polarization and color characteristics. Accordingly, the prior art is in such a state that acceptable characteristics are hardly maintained by using a multi-layered membrane, normally including 10 to 20 layers.